1. Field of the Invention
This invention relates generally to tractors for moving equipment within passages and, more particularly, to a hydraulically powered tractor having an improved valve system.
2. Description of the Related Art
The art of moving equipment through vertical, inclined, and horizontal passages plays an important role in many industries, such as the petroleum, mining, and communications industries. In the petroleum industry, for example, it is often necessary to move drilling, intervention, well completion, and other forms of equipment through boreholes drilled into the earth.
One method for moving equipment through a borehole is to use rotary drilling equipment. In traditional rotary drilling, vertical and inclined boreholes are commonly drilled by the attachment of a rotary drill bit and/or other equipment (collectively, the “Bottom Hole Assembly” or BHA) to the end of a rigid drill string. The drill string is typically constructed of a series of connected links of drill pipe that extend between ground surface equipment and the BHA. A passage is drilled as the drill string and drill bit are together lowered into the earth. A drilling fluid, such as drilling mud, is pumped from the ground surface equipment through an interior flow channel of the drill string to the drill bit. The drilling fluid is used to cool and lubricate the bit, as well as for removing debris and rock chips from the borehole. The drilling fluid returns to the surface, carrying the cuttings and debris, through the annular space between the outer surface of the drill pipe and the inner surface of the borehole. As the drill string is lowered or raised within the borehole, it is necessary to continually add or remove links of drill pipe at the surface, at significant time and cost.
Another method of moving equipment within a borehole involves the use of downhole tools commonly referred to as “tractors.” A tractor is capable of gripping onto the borehole and thrusting both itself and other equipment through it. A self-propelled tractor of this type may be used for pushing and pulling adjoining equipment through inclined or horizontal boreholes. Tractors can be attached to rigid drill strings or may be used in conjunction with coiled tubing equipment.
Coiled tubing equipment generally includes a non-rigid, compliant tube, referred to herein simply as “coiled tubing,” through which operating fluid is delivered to the tractor. The operating fluid can provide hydraulic power to propel the tractor and the equipment and, in drilling applications, to lubricate the drill bit. In such systems, the operating fluid may also provide the power necessary for enabling the tractor to grip the inner surface of the borehole. In comparison to rotary equipment, the use of coiled tubing in conjunction with a tractor is generally less expensive, easier to use, less time consuming to employ, and provides more control of speed and downhole loads. In addition, due to its greater compliance and flexibility, the coiled tubing permits the tractor to negotiate sharper turns in the borehole than rotary equipment.
Due to their versatility, self-propelled tractors may be used in a wide variety of applications. For example, a tractor may be used for well completion and production work for producing oil from an oil well, pipeline installation and maintenance, laying and movement of communication lines, well logging activities, washing and acidizing of sands and solids, retrieval of tools and debris, and the like. One type of tractor comprises an elongate body securable to the lower end of a drill string. The body may include one or more joined shafts attached to a control assembly housing or valve system.
Tractors generally include at least one anchor or gripper assembly adapted to grip the inner surface of the borehole. When the gripper assembly is actuated, hydraulic power from operating fluid may be used to propel the body axially through the borehole. The gripper assembly is longitudinally movably engaged with the tractor body, so that the body and drill string can move axially through the borehole while the gripper assembly is anchored to the inner surface of the passage. Several embodiments of a fluid-actuated gripper assembly are disclosed in U.S. Pat. No. 6,464,003 to Bloom et al. In one highly effective embodiment, the gripper assembly includes a plurality of flexible toes that expand radially outward by the interaction of ramps and rollers to engage, and thereby grip, the inner surface of the passage.
Tractors are commonly configured with two or more sets of gripper assemblies, which provide the ability to have at least one gripper anchored to the borehole at all times. This configuration permits the tractor to move in a substantially continuous manner within the passage. Forward longitudinal motion (unless otherwise indicated, the terms “longitudinal” and “axial” are herein used interchangeably and refer to the longitudinal axis of the tractor body) is achieved by powering the tractor body forward with respect to an actuated first gripper assembly (a “power stroke” with respect to the first gripper assembly), and simultaneously moving a retracted second gripper assembly forward with respect to the tractor body (a “reset stroke” of the second gripper assembly). At or near the completion of the power stroke with respect to the first gripper assembly, the second gripper assembly is actuated and the first gripper assembly is retracted. Then, the tractor body is powered forward while the second gripper assembly is actuated (a power stroke with respect to the second gripper assembly), and the retracted first gripper assembly executes a reset stroke. At or near the completion of these respective strokes, the first gripper assembly is actuated and the second gripper assembly is retracted. The cycle is then repeated. Thus, each gripper assembly operates in a cycle of actuation, power stroke, retraction, and reset stroke, resulting in longitudinal motion of the tractor.
A number of highly effective tractor designs utilizing this configuration are disclosed in U.S. Pat. No. 6,003,606 to Moore et al., which discloses several embodiments of a tractor known as the “Puller-Thruster Downhole Tool;” U.S. Pat. No. 6,241,031 to Beaufort et al.; and U.S. Pat. No. 6,347,674 to Bloom et al., which discloses an “Electrically Sequenced Tractor” (“EST”).
As discussed above, the power required for actuating the gripper assemblies, longitudinally thrusting the tractor body during power strokes, and longitudinally resetting the gripper assemblies during reset strokes may be provided by pressurized operating fluid delivered to the tractor via the drill string. Typically, one or more flow control devices, such as valves, are provided within the tractor body for distributing the operating fluid to the tractor's gripper assemblies, thrust chambers, and reset chambers.
Some types of tractors, including several embodiments of the Puller-Thruster Downhole Tool, are entirely hydraulically powered. Pressure-responsive valves typically shuttle between various positions based upon the pressure of the operating fluid in various locations of the tractor. In one configuration, a pressure-responsive valve may take the form of a spool valve that is exposed on both ends to different fluid chambers or passages. As a result, the valve position depends on the differential pressure between the fluid chambers. Fluid having a higher pressure in a first chamber exerts a greater force on the valve than fluid having a lower pressure in a second chamber, forcing the valve to one extreme position. The valve moves to another extreme position when the pressure in the second chamber is greater than the pressure in the first chamber. Another type of pressure-responsive valve takes the form of a spring-biased spool valve having at least one end exposed to fluid. The fluid pressure force is directed opposite to the spring biasing force, so that the valve is opened or closed only when the fluid pressure exceeds a threshold value.
In other configurations, tractors may be provided with one or more valves that are controlled by electrical signals sent from a control system at the surface or even on the tractor itself. For example, the aforementioned EST includes both electrically controlled valves and pressure-responsive valves. The electrically controlled valves are controlled by electrical control signals sent from a controller housed within the tractor body. For drilling operations, the EST may be preferred over all-hydraulic tractors because electrical control of the valves permits very precise control over important drilling parameters, such as speed, position, and thrust.
In contrast, all-hydraulic tractors, including several embodiments of the Puller-Thruster Downhole Tool, are generally preferred for so-called “intervention” operations. As used herein, the term “intervention” refers to re-entry into a previously drilled well for the purpose of improving well production, to thereby improve fuel production rates. As wells age, the rate at which fuel can be extracted therefrom diminishes for several reasons. This necessitates the “intervention” of many different types of tools. Hydraulic tractors are generally preferred over electrically controlled tractors for intervention operations because hydraulic tractors are less expensive to operate and intervention operations do not require precise control of speed or position.
Tractors used in combination with coiled tubing equipment are particularly useful for intervention operations because, in many cases, the wells were originally drilled with rotary drilling equipment capable of drilling very deep holes. It is more expensive to bring back the rotary equipment than it is to bring in a coiled tubing unit. However, in many situations, the coiled tubing unit may not be capable of reaching extended distances within the borehole without the aid of a tractor. The tractor is particularly useful for reaching locations within inclined or horizontal boreholes.
Those skilled in the art appreciate that tractors of the type generally described above may be exposed to a wide variety of different conditions. For example, depending on the particular application, the pressure, weight, and density of the operating fluid may vary significantly. Furthermore, the shape and angle of the borehole may vary. In addition, the weight of the equipment that the tractor must pull and/or push will differ with the particular application.